The subject invention concerns a device in an alarm system and is designed to sense the entrance of an article equipped with a particular marker into a magnetic field generated for the purpose of this sensing operation. The marker comprises at least one strip of a material having ferromagnetic soft properties and sensing means are provided to sense the characteristic tones generated by the ferromagnetic soft strip material in the magnetic field. Adjacent each strip of ferromagnetic soft material there is arranged a second strip of a material having properties allowing it to be magnetized but which material is non-magnetized in its untreated state. A means is designed to neutralize the marker, ensuring that the alarm remains inactivated when the marker passes through the magnetic field. The device is intended for use in alarm systems in in retail stores, libraries and similar premises to prevent shop-lifting and thefts.
Shoplifting and thefts from shops and stores are quite common. The waste in shops and stores in Sweden is estimated to quite considerable amounts per household and year. The shoplifting may be of an unplanned and impulsive type but also be well-planned and carried out on a large scale.
Alarm devices of the type defined in the introduction are well known and used in various applications. These alarm devices are designed to make use of the fact that when a piece of metal (preferably of a so-called high-permeability material) enters a sinusoidally varied magnetic field a voltage is induced in coils arranged in the vicinity of the applied magnetic field. Each metal which enters the magnetic field generates a voltage which is characteristic of that metal. This voltage consists of a basic component and a number of characteristic harmonics. In alarm devices of the kind described above the harmonics generated by a particular material included in the marker of an article alerts the alarm. An alarm device functioning in accordance with this principle is described in FR No. 763 681.
In GB No. 1 538 385 is described an example of an alarm device functioning on a somewhat different principle from the one described above. Two magnetic fields are established at the exit from a shop or similar premises. These two magnetic fields oscillate at different frequencies and when a marker including ferromagnetic soft material is introduced into these magnetic fields it will give off a tone of a frequency representing the difference between the frequencies with which the two magnetic fields oscillate. Sensing means are set to detect the tones of this frequency and upon detection thereof trigger off the alarm.